The present invention relates to mobile communications systems. In particular, the invention relates to a method and apparatus for adapting the handoff threshold in such systems.
Channel handoff or handover is an essential technology in cellular communication systems. It allows the mobile unit user to move from cell to cell within system coverage areas without breaking an ongoing telephone call and without loosing synchronization with the system when the mobile is on-hook. There are mainly two type of inter-cell handoffs: hard handoff and soft handoff.
In a hard handoff case, as illustrated in FIG. 1, the base stations in the system are periodically scanning channels in their receiving frequency band and sending the scanning results back to a radio network controller (RNC). In some systems, the mobile units are also periodically scanning all channels in their receiving bands and using idle time slots to send the scanning results to the controller through their serving base stations. The scanning information contains radio signal strength indicators (RSSI), bit error rates other channel information. The controller processes channel information from all of its base stations and mobile units.
Assuming that the mobile unit 2 in FIG. 1 is currently served by BS-1 and cell 3, the movement of the mobile unit towards BS-2 and cell 4 creates a candidate for handoff to BS-2 and cell 4. The RNC evaluates the averaged RSSI difference between the mobile unit 2 and both BS-1 and BS-2. If following condition is met:
RSSIBS-2xe2x88x92RSSIBS-1xe2x89xa7ThresholdHandoff,xe2x80x83xe2x80x83(Eq. A)
the RNC will allocate an unused channel in BS-2 and handoff the physical connection of the mobile unit 2 from BS-1 to BS-2.
Because a minimum amount of time is needed to execute the handoff process, an ongoing call will experience a short break in transmission. The short break experienced by the user is an inconvenience, and has a bearing on overall service quality perception by the user. Furthermore, data transfer may be partially corrupted during the break.
In a soft or xe2x80x9cseamlessxe2x80x9d handoff case, a mobile has two or more links with different base stations that are involved in the handoff process. For example, in FIG. 1, the old link between the mobile unit and BS-1 will be released only when a new radio link between the mobile unit and BS-2 is already established. Because of this, a mobile user will not notice the handoff break during an ongoing call. Equation A is a commonly used handoff criterion for soft handoff.
Digital wireless systems exist in which the handoff decision is made by mobiles. Typical examples are DECT and PWT-based systems. The DECT/PWT system is based on MC/TDMA/TDD technology. Because of TDD, the uplinks and downlinks use the same frequency carrier. In such a system, the handoff decision is made in the mobile unit. Soft handoff is used when the mobile user moves from cell to cell in the system coverage areas. These inter-cell handoffs also use the condition of Equation A as the triggering criterion for handoff.
Conventional inter-cell handoff processes (including hard handoff and soft handoff) normally comprise four steps: 1) Scanning the radio environment RSSI""s; 2) processing the scanned RSSIs (RSSI is sometimes averaged with previously scanned values); 3) checking the handoff threshold (RSSI(anyxe2x88x92BS)xe2x88x92RSSI (Active_BS)xe2x89xa7ThresholdHO); and 4) executing the handoff procedure.
The main shortcoming of this procedure is that a fixed handoff threshold is often used in conventional systems. This mechanism operates effectively in high power, macro-cellular systems with few, widely spaced base stations. More recently, however, users operate mobile units within densely populated areas or xe2x80x9cpico-cellular environmentsxe2x80x9d which exist in urban centers, cities, buildings and arenas. In such environments, the system is merely capacity limited. Thus, to meet system capacity needs, an area will be covered by more base stations than what signal strength requires. When mobile users move around the system coverage areas, a high number of system-wide inter-cell handoffs may occur.
FIG. 2 shows a typical pico-cellular business wireless system configuration 5. In this system, mobile users are allowed to move about the system coverage area when receiving incoming calls and making outgoing calls. When a mobile-A user moves from cell 1 to cell 2, and if the condition of Equation A is fulfilled, an inter-cell handoff will occur if mobile-A user is participating in an ongoing call. If the mobile-A user is not participating in a call, an inter-cell standby channel switch will occur. In either case, a certain amount of information processing and signaling exchange will take place to handle this event, thereby adding to the processing load of the system.
A similar problem is illustrated in the block diagram of FIG. 3. Assuming a fixed handoff threshold in this cellular system 8, there are three potential inter-cell handoffs associated with the walking path 9 of user A if the system utilizes a relatively low handoff threshold. If the mobile unit is sufficiently covered by BS-1 during the entire walking path, the radio link between the mobile and BS-1 can be good enough to provide a high-quality connection throughout the path 9. These three potential inter-cell handoffs therefore become unnecessary and undesired.
This problem cannot be resolved by designing a high handoff threshold into the system. In a capacity limited system, radio link quality will be determined by an instantaneous signal-to-noise ratio. In FIG. 3, for example, if the mobile is in position B, a high threshold would require the mobile to maintain the radio link with remote BS-1 without handing off to BS-2. If the interference level is very high due to telephone traffic and other unsynchronized wireless systems operating in the same frequency band, the signal-to-noise ratio of the radio link between the mobile unit and BS-1 might not be sufficient to provide acceptable voice or data quality or, more seriously, cause a call drop.
Thus, optimization of both processing load and system service quality is difficult when the mobile utilizes a fixed inter-cell handoff threshold in an increasingly dynamic radio environment.
The solution to resolve this problem is to use an adaptive inter-cell handoff threshold. Often, however, these prior art threshold modification methods relied mostly on variations in signal strength of neighboring cells and the probability of good quality associated therewith, rather than evaluating the quality of the present communication link.
The present invention is directed to a method and apparatus of adapting a threshold for channel handoff and channel switch in cellular, PCS and business wireless communication systems. When the interference level in a radio environment is low, the proposed method and apparatus solves the problems of unnecessary inter-cell handoff when the mobile unit user is moving within the system coverage areas. When the interference level is high, the proposed method and apparatus provides a new radio channel or cell to facilitate the access to stronger base stations in the coverage area.
In one aspect of the present invention, the method includes the steps of evaluating the signal quality for the communication and lowering the dynamic threshold to encourage handoff if the signal quality indicator is lower than a preset quality threshold. The preset quality threshold represents a minimum acceptable signal quality level. The dynamic threshold is raised to discourage handoff if the signal quality indicator of the communication is at or near a maximum signal quality representing a signal having few errors.
In another aspect of the present invention, a method of handing off a cellular communication between a mobile unit and a cell within a cellular network is provided. The communication has a signal strength and signal quality measurable at the mobile unit. The method includes the steps of scanning the radio environment at the mobile unit to evaluate signal strength and signal quality conditions of alternative channels within the mobile unit cell or in neighboring cells, selecting a dynamic threshold corresponding to a signal strength value in accordance with the signal quality of the present communication and whether the signal quality is below or at a maximum or minimum value, and handing off the communication to a neighboring cell if the signal strength of an alternative channel measured at the mobile unit exceeds the dynamic threshold.
In yet another aspect of the present invention, a method is provided for adjusting a dynamic threshold for allowing handoff of a mobile unit communication. The method includes the steps of determining a signal strength indicator and a signal quality indicator for the communication, and adjusting the dynamic threshold in accordance with conditions of the communication at the mobile unit, including the link quality and whether the mobile unit is presently off-hook or in standby mode.
In yet another aspect of the present invention, an apparatus for handing off a cellular communication between a mobile unit and a base station within a cellular network is provided wherein the communication has a signal strength and signal quality measurable at the mobile unit. The apparatus includes means for scanning the radio environment at the mobile unit to evaluate signal strength and signal quality conditions of alternative channels within the mobile unit""s cell or in neighboring cells, means for determining whether the mobile unit is participating in an off-hook communication or a standby communication, means for selecting a dynamic threshold corresponding to a signal strength value in accordance with the signal quality of the present communication and whether the signal quality is below or at a maximum or minimum value, and means for handing off the communication to an alternative channel if the signal strength of the alternative channel measured at the mobile unit exceeds the dynamic threshold.
Thus, by utilizing the present invention, the dynamic handoff threshold is adapted in accordance with the link quality of the present communication channel. If the present communication has acceptable quality and strength, the present invention modifies the threshold to discourage handoff even though stronger neighboring cells or channels may exist. Conversely, if the present communication is degrading in quality, the handoff threshold is decreased to encourage handoff to a neighboring cell likely to establish a high-quality link. Through the use of the present invention, the utilization of system processing resources and the overall quality of the system will be improved.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The invention, together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.